An aluminum alloy is used in a wide variety of fields as a lightweight material, and various aluminum alloys that are suitable for casting have been developed.
A gravity die casting process, a low pressure die casting process, a high pressure casting process, and the like are known as a casting process. A die casting process is classified as a high pressure casting process, and achieves high productivity.
The die casting process injects aluminum alloy molten metal into a die (mold) at a high speed under high pressure to produce a cast member. A JIS (Japanese Industrial Standards) ADC12 aluminum alloy is widely applied to automotive parts and the like since a dense and high-strength cast structure can be obtained.
The ADC12 aluminum alloy is an Al—Si—Cu—Fe—Mg—(Zn)-based aluminum alloy, and exhibits high strength and high yield strength in an as-cast state (i.e., without heat treatment).
However, since the ADC12 aluminum alloy exhibits low ductility, it is difficult to apply the ADC12 aluminum alloy to parts for which high toughness is required.
In particular, a reduction in weight is strongly desired in the fields of airplanes, rail vehicles, and automobiles, and a casting aluminum alloy that exhibits high ductility and can also be applied to structural members has been desired.
A hypo-eutectic Al—Si—Mg-based alloy and a hypo-eutectic Al—Mg—Si-based alloy have been studied as an aluminum alloy that exhibits high ductility (high toughness) and high strength.
Note that the term “Al—Si—Mg-based alloy” refers to an aluminum alloy in which the Si content (that is higher than the content of each component added to Al) is higher than the Mg content, and the term “Al—Mg—Si-based alloy” refers to an aluminum alloy in which the Mg content is higher than the Si content.
Typical examples of the Al—Si—Mg-based alloy include an AA365 alloy that is specified in the United States standards.
The AA365 alloy has a relatively high Si content (8 to 12 mass %) and a low Mg content (0.6 mass % or less). Since the AA365 alloy exhibits high ductility, but exhibits insufficient strength, it is necessary to perform heat treatment (e.g., T5 heat treatment) after the die casting process, whereby an increase in cost occurs. Moreover, a change in dimensions or shape may easily occur during the heat treatment.
An Al—Mg—Si-based alloy that has a high Mg content (2 to 8 mass %) and a low Si content (0.5 to 3 mass %) has been proposed.
However, this Al—Mg—Si-based alloy has a problem in that shrinkage may occur during solidification, and cracks (casting cracks) may easily occur during casting.
JP-A-2009-108409 discloses an Al—Mg-based aluminum alloy that includes 2.5 to 5.0 mass % of Mg, 0.3 to 1.5 mass % of Mn, and 0.1 to 0.3 mass % of Ti, and exhibits excellent toughness, the Al—Mg-based aluminum alloy preferably further including 0.2 to 0.6 mass % of Si and 0.005 to 0.05 mass % of Sr.
The Si content in the casting alloy disclosed in JP-A-2009-108409 is set to be as low as 0.2 to 0.6 mass % in order to suppress the needle-like growth (crystallization) of Mg2Si compounds (see paragraphs [0026] to [0028] of JP-A-2009-108409).
JP-T-2010-528187 discloses an aluminum alloy that is designed to reduce hot tearing sensitivity, and includes 0.01 to 0.025 mass % of Sr, and TiB2 in an amount corresponding to 0.001 to 0.005 mass % of B.
In JP-T-2010-528187, Sr is added to promote the formation of spheroidal crystal grains in the α-Al crystal grains through a synergistic effect with TiB2 (see paragraph of JP-T-2010-528187).